<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    //小于10输出0几，大于等于10直接输出
    let num = prompt('请输入一个数字')
    num = num < 10 ? 0 + num : num
    // num = num >= 10 ? num : 0 + num
    alert(num)
  </script>
</body>

</html>